Jacob Nielsen's Seven Deadly Sins of Web Design

Long download times. More critical than ever. You have 10 seconds before users get bored and leave.

Lack of navigation support. Don't assume users know as much about your site as you do.

Outdated information. Hire a Web garderner. Maintenance is a cheap way to enhance the content of a
web site.

Frames. They break the fundamental user model of the web page. URLs don't work: You cannot bookmark
the page and return to it.

Annoying animation. Mainstream users care more about useful content.

Orphan pages. Pages that lack logos, links or other hints to indicate what site they are part of.
Always indicate what site the page is on and provide a link to the home page.

Long, scrolling pages. No longer a catastrophe, but still a problem. People don't like to scroll.

(from Technology Review, Sept/Oct. 1998, p. 74)